Process for producing selenium dioxide



United States Patent PROCESS FOR PRODUCING SELENKUM DISXIDE Edward S.Roberts, 874 Woodward Ave., Ridgewood, N.Y., and Ludwig .l. Christmann,9 Center Knolls, Bronxville, N.Y.

No Drawing. Application May 11, 1962, Ser. No. 194,200, which is acontinuation of applications Ser. No. 4,211, Jan 25, 1960, Ser. No.5,360, Jan. 29, 1960, Ser. No. 15,049, Mar. 15, 1960, Ser. No. 19,460,Apr. 4, 1960, and Ser. No. 176,101, Feb. 27, 1962. Divided and thisapplication June 10, 1965, Ser. No. 473,268

7 Claims. (Cl. 23139) This is a-division of application Serial No.194,200, filed May 11, 1962, which is a continuation of applicationsSerial No. 4,211, filed January 25, 1960, and now abandoned; Serial No.5,360, filed Jaunary 29, 1960, and now abandoned; Serial No. 15,049,filed March 15, 1960, and now abandoned; Serial No. 19,460, filed April4, 1960, and now abandoned and Serial No. 176,101, filed February 27,1962, and now abandoned; all by the present in ventors, which last-namedapplication was a continuationin-part of application Serial No. 7,533,filed February 9, 1960, and now abandoned.

The present invention is directed in one embodiment to the production ofoxidized organic compounds.

One aspect of the invention involves oxidation of alkylaromaticcompounds to aromatic carboxylic acids. Another aspect of the inventioninvolves oxidation of polycyclic aromatic compounds to aromatic ketones.

In one embodiment, the invention i directed to the oxidation ofacenaphthene and substitution products thereof wherein the substituentsdo not interfere with the oxidation. More specifically, the inventionincludes the production of quinones and polycarboxylic acids from thestarting materials.

Heretofore, the quinone of acenaphthene was made by the reaction ofacenaphthene with alkyl nitrite or nitrosyl chloride to form the oxime,which was then hydrolyzed to form the quinone. This process required anumber of steps, the use of expensive reagents, and the efiiciency ofthe conversion was relatively low.

In another prior art process, the acenaphthene was treated with sodiumbichromate in acetic acid to form the quinone. But it suffered from thedisadvantages that the medium was corrosive, the reagents wereexpensive, the yield was low and side reaction products were formed. Ina modification of this process, the reactants in water solution wereheated in an autoclave under substantial pressure; this had theadditional disadvantage that expensive equipment was necessary andworking under pressure had an element of danger. Another difiiculty withoxidation reactions involving complex organic molecules is that readilycondensible intermediate products are fre quently formed in the firststages and undesirable condensation products are produced, which notonly interfere with the desired reaction but also Waste valuablematerials and make purification of the desired product more difficult.

The present invention is intended and adapted to overcome thedifficulties and disadvantages inherent in prior processes for producingoxidation products of acenaphthenes, it being among the objects thereofto devise a process for the above purpose which is simple and highlyeffective.

It is also among the objects of the invention to treat acenaphthenes soas to produce both quinones and poly ice enter into or interfere withthe desired reactions. Applicants have discovered that it is possible tooxidize the methylene (CH groups by means of nitrogen dioxide (NO toconvert them essentially to CO groups. To accomplish this the startingmaterial in solution in suitable solvents is heated to a sufiicientlyelevated temperature above that at which N 0 is largely dissociated, andpassing N0 through the solution, preferably with stirring. The N0 isreduced to NO and the acenaphthene is oxidized to quinone. It is notedthat the red-brown color of the N0 fades out, and the NO formed in thereaction together with water of reaction are removed, the watercondensed and the NO reoxidized for return to the reaction.

Applicants have found that for practical purposes the temperature of thereaction should be above about C. Preferably, temperatures in theneighborhood of 225 C. are used, but the temperature should be keptbelow the decomposition point of the substances involved in thereactions. Also, it is preferable that the concentration of acenaphthenebe low, as this insures that loss of ingredients by side reactions ispractically eliminated.

Such solvents as are suitable for the process must be insert to nitrogenoxides at the elevated temperatures necessary for this process. Theboiling points thereof should be substantially above the temperatures ofthe reaction at the pressures used in the operation. Relatively fewsolvents meet these requirements and, after considerableexperimentation, it was found that the following solvents, which show anegligible reaction with N0 are suitable:

Mixed tetrachlorobenzenes Trichlorobenzene DichlorobenzeneMonochlorobenze Other solvents, which show some reaction with N0 but arestill usable, are as follows:

Nitrobenzene Diphenyl ether Chlorinated diphenyls While the above inertsolvents have been found suitable, and are the best so far found, othersolvents may also be used. I

When oxidizing 3,4 acenaphthene dicarboxylic acid for instance, a chargeof this substance amounting to 5% to 10% by weight of an inert solventsuch as trichlorobenzene may be added to the solvent and heated to C.before introduction of the N0 With provision for fine dispersion of theN0 as it is introduced and with sufficient time of contact of the gaswith the liquid, reaction of the N0 to NO is virtually complete by thetime the gas leaves the reactor until nearly all of the 3,4 acenaphthenedicarboxylic acid is oxidized to the quinone. The exit gases from thereactor contain virtually no N0 or free 0 This allows the H 0 arisingfrom the oxidation of the acenaphthene derivative to be removed bycondensation or condensation followed -by drying with silica gel withoutloss of N0 or NO.

After drying of this gas stream, air or 0 is introduced and mixed withit and the water-free NO-O mixture passed into a silica gel bed whichcatalyses the oxidation of NO to N0 and adsorbs the N0 The N0 is thenrecovered from this silica gel bed by heating the bed, driving off theN0 as a gas for recycle to the oxidation process. This means thatessentially, atmospheric oxygen is used as the oxidant and that N0simply serves as a carrier of oxygen.

When oxidizing acenaphthene to the quinone by N0 it is desired to keepthe acenaphthene concentration in aaeseea the solvent low by adding theacenaphthene continuously at a rate proportioned to the N addition closeto the theoretical proportion of one of acenaphthene to 4 N0 by weight.By proper selection of these rates according to the equipment used, itis here also possible to keep the quantity of N0 in the exit gases verylow in relation to the NO in these gases. This allows the NO to bereoxidized to N0 and this N0 to be recovered and recycled as describedabove.

Acenaphthene quinone and acenaphthene quinone dicarboxylic acid or theimide of this dicarboxylic acid are important intermediates for theproduction of 1,8 naphthalene dicarboxylic acid (naphthalic acid) andl,4,5,8 naphthalene tetracarboxylic acid, used in making dyestuffs andpigments.

The quinones so formed are then converted into the correspondingcarboxylic acids or their anhydrides. It has been found by applicantsthat this may be accom plished by suspending the quinone in a relativelyweak caustic soda solution containing slightly in excess of two mols ofcaustic per mol of acenaphthene quinone and then oxidized with NaOCl insolution in amount slightly in excess of one m-ol per mole ofacenaphthenequinone. The same operation may be carried out onacenaphthene quinone dicarboxylic acid and 1,4,5,8 naphthalenetetracarboxylic acid produced therefrom.

The following are specific examples which will illus trate the inventionwithout limiting the scope thereof.

Example 1 1400 cc. of trichlorobenzene were placed in a 3 liter resinflask provided with a center downdraft high speed propeller agitatornear the bottom and with four vertical baflies around the periphery atthe bottom and with a gas inlet tube leading down to a point just abovethe propeller.

Two hundred grams of acenaphthene were dissolved in 600 cc. oftrichlorobenzene making approximately 780 cc. of solution. Sixty cc. ofthis solution were added to the resin flask and heated to 190 C. underagitation. N0 was introduced through the gas inlet tube at "a rate ofabout 1.5 g./min. and the temperature maintained at 190 to 195 C. Theremaining acenaphthene solution was dripped into the resin flask at sucha rate as to give nearly complete elimination of N0 in the exist gasesas judged by the color of these gases. The exit gases were cooled in anair condenser and the condensate collected. After 46 g. of watercondensate were collected the N0 was shut off and the N0 was purged fromthe system with N The solvents of the resin flask were cooled and 138 g.of solids were separated from the solution. These solids were suspendedin strong caustic and oxidized with H 0 The product of this oxidationwas neutralized with HCl after dilution and the solids separated anddried at 115 C. for 48 hours. This product showed a neutralizationequivalent of 98.5 or approximately that of naphthalic anhydride.

Example 2 1400 cc. of mixed tetrachlorobenzenes were put in a 3 literresin fiask described in Example 1. Two hundred grams of acenaphthenewere dissolved in 600 cc. of the tetrachlorobenzene. Sixty cc. of thissolution were added to the resin flask and heated to 210 C. N0 was thenadmitted through the gas inlet tube at a rate of about 1.8 g./rnin. Theremaining acenaphthene solution was continuously dripped in over aperiod of about 3 hours or at such a rate as to give virtually completeelimination of N0 in the exit gases. The temperature of the contents ofthe flask was allowed to go to about 225 C. during this addition. Afterpurging the flask and its contents with N the contents were removed,cooled, and the solids filtered oif. The filter cake was reslurried in1,000 cc. of isooctane, brought to a boil, cooled to room temperature,

ltered and washed with an additional 500 cc. of isooctane to remove thetetrachlorobenzenes.

The solids were then dried. The weight recovered was 191 g. of driedsolids. Thirty grams of these solids were dissolved in cc. of hotnitrobenzene and recrystallized by cooling, separated by filtration,washed with methanol to remove the nitrobenzene and dried. This gave 26g. of dry crystalline solids which were identified as acenaphthenequinone by the melting point.

The filtrate from the N0 oxidation described above was returned to the 3liter resin flask with suflicient additional tetrachlorobenzene to makeup the volume, and an additional 200 g. of acenaphthenewas oxidized inthe same way as described above. This procedure was repeated five moretimes with all the results summarized as follows:

nitrobenzene Weight in grams 339 g. taken from a composite of runs 4, 5and 6 were dissolved in 1130 cc. of hot nitrobenzene and recrystallizedby cooling. The solids were separated by filtration, washed with 500 cc.of methanol to remove the nitrobenzene, dried and weighed. 272 g. ofrecrystallized quinone were recovered.

Example 3 A sample of the above 272 g. of acenaphthene quin-one wasoxidized to naphthalic acid as follows:

60 g. of quinone was mixed with 30 g. of NaOH and 560 cc. of 5.25% NaOClsolution and heated to a boil slowly, and diluted to 3000 cc. giving apale yellow solution. Na S O was added until a starch-iodide paper testshowed no excess NaOCl. The solution was acidified with HCl, filtered,and the solids Washed and dried over night at C. 61 g. of dried whitesolids were recovered, having a neutralization equivalent of 99.3 orthat of naphthalic anhydride.

Example 4 100 g. of 3,4 acenaphthene dicarboxylic acid were dissolved in2000 cc. of trichlorobenzene in a 3 liter resin flask equipped asdescribed in Example 1. This solution was heated to C. and oxidized withN0 added at a rate of about 1.8 g./min. until the exit gases were thesame color as the inlet N0 The reaction mixture was cooled, filtered andthe solids washed with isooctane and dried.

87 g. of dry solids were recovered. 27 g. sample of this cake was mixedwith 16 g. of NaOH and 260 cc. of 5.25% aqueous NaOCl solution heatedslowly to boiling, diluted to 3 liters and the excess NaOCl removed byaddition of Na S O as indicated by a starch-iodide test. The solutionwas then acidified with HCl, the solids filtered 0d and washed withwater and dried. The dry cake weighed 22 g. and showed a neutralizationequivalent of 71.7 which lies between the tetracarboxylic acid ofnaphthalene and the dianhydride of this acid.

In another embodiment, the invention is directed to the treatment ofalkyl naphthalenes, and more particularly to methyl naphthalenes, saidtreatment being for the purpose of producing oxidation products thereof,specifically carboxy naphthalenes.

In a prior art process Z-methyl naphthalene was treated with acetylchloride in the presence of d-ichloroethane as a solvent and with boronfluoride as catalyst, whereby 2-methy1-6-acetyl naphthalene was formed.Then the.

5 product was treated with dilute nitric acid at elevated temperaturesto form 2,6-naphthalic acid. This process had a number of deficiencies.There were a number of steps and intermediate operations necessary whichresulted in losses and low yields. Side reaction products were formedand in no case was the final product largely the 2,6-naphthalic acid.The conditions of the reaction were critical; if the concentration ofthe nitric acid was too high, nitrated by-products were formed; if theexcess of nitric acid was too high, degradation products of oxidationwere formed; it the temperature was not sufliciently high, little or no2,6-naphthalic acid was formed, and if the temperature was too high,heat decomposition was likely to take place.

It was also proposed to oxidize 1,6-dimethyl naphthalene by the use ofpotassium ferricyanide together with potassium hydroxide in two stagesat elevated temperatures. This process required large amounts ofreactants and the yield was very low. The resulting 1,6-naphthalenedicarboxylic acid was contaminated with large amounts of othersubstances so that recovery and purification thereof was quitediflicult.

The present invention is intended and adapted to overcome thedisadvantages inherent in prior processes of the type described, itbeing among the objects thereof to provide a process which is relativelysimple and is highly effective in obtaining a high yield of oxidationproduct, and with recovery of the oxidant for reuse.

It is also among the objects of the invention to oxidize polyalkylnaphthalenes to produce polycarboxy derivatives thereof, particularlythe di-carboxy compounds.

It is further among the objects of the invention to provide a processwhich results in products of relatively high purity and the isolation ofwhich is simple.

In practicing the invention, there is utilized N as the oxidizing agentand in the gaseous form. The alkyl naphthalene is dissolved in asuitable solvent which is inert to the N0 at the temperatures used. Suchsolvents as the chlorobenzenes have been found suitable; while themonoto hexa-chlorobenzenes are useful, it is preferable to use the ditotetra-chloirobenzenes as being eminently suitable. Other solvents havingthe desired properties may also be used.

The temperature of the reaction is above that at which N 0 is largelydissociated to N0 as the efliciency of the reaction is dependent on N0Such temperatures are about 125140 C., but it is desirable to operate athigher temperatures to speed up the reaction, the maximum temperaturebeing below that at which decomposition of products or reactants takesplace. It has been found that temperatures above about 180 and up toabout 225 C. are quite satisfactory.

Under the preferred conditions of operation, the N0 is passed throughthe solution of alkyl naphthalene at such a rate that it issubstantially completely reduced to NO. This has the important advantagethat it can be reoxidizcd to N0 very readily and by a simple operation,tor recycling without loss of nitrogen oxides.

The following are specific examples of the operation of the presentinvention:

Example 5 The apparatus consists of a 3-liter resin flask fitted with astirrer and having baflles along the sides to give effective agitation.inlets are provided for both N0 and for the solution of the alkylnaphthalene, reaching to near the bottom of the flask. A reflux or othercondenser attached to the top returns condensed vapors (other thanwater) to the flask. An overflow outlet near the top perrnits the exitof the oxidized mixture for further process- 200 g. of 1,6-dimethylnaphthalene were dissolved in 2000 cc. of trichlorobenzene. The flaskwas heated to 160 C. and N0 was introduced into the bottom of the flaskat the rate of 1.65 g. per minute and the temperature was raisedgradually over one-half hour to 190 C., where 6 the temperature was heldfor 1.5 hours while continuing the flow of N0 Then the rate of flow ofN0 was increased to 2 g. per minute for an additional 1.5 hours. Duringthe first two hours the eifluent gases were a very pale yellow showingsubstantially complete reduction to NO, after which the color slowlydeepened somewhat.

A total of 40 g. of water in vapor form was produced in the reaction andwas condensed from the exit gases. The reaction mixture was cooled, thesolid reaction product was separated therefrom by filtration, washedwith naphtha to free it from the solvent, and dried. The dried solidsweighed g. and showed a N.E. (neutralization equivalent) of 131,indicating the presence of dibasic acid.

Example 6 Into the flask equipped as in Example 5, there was introduced2000 cc. of trichlorobenzene and 10 g. of selenium (Se). The flask washeated to 105 C. and N0 was introduced to oxidize the Se to SeO 200 g.of 1,6-dimethyl naphthalene were dissolved in the solution withstirring, causing the solid Se0 to dissolve forming a clear redsolution. N0 was introduced at the rate of 2 g. per minute whilemaintaining the temperature between and 198 C. until 41 cc. of watervapor was produced, which was condensed to separate the water from theexit gases, which remained practically colorless at the end of thereaction.

The contents of the flask were cooled, the solids removed therefrom byfiltration, washed with isooctane to free them from the solvent, thenwashed with water, and dried. The product weighed 224 g. and showed aNE. of 117. To convert the intermediate product to acid or anhydride, itwas treated with NaOCl in NaOH aqueous solution, the excess NaOClremoved by the addition of Na S O as shown by the starch-iodide test,and acidified to precipitate the substantially pure acid product, whichshowed a NE. of 108.

Example 7 .200 g. of 2,6-dimethyl naphthalene were dissolved in 2000 g.of trichlorobenzene in the flask described in Example 5, and heated to185 C. N0 was introduced at the rate of about 2 g. per minute, thetemperature being raised to 193 C. over one hour, then lowered to 191 C.over the remaining 2.5 hours.

The exit gases were colorless over the first hour and then graduallybecame reddish by the end of the reac tion. 41 g. of water were obtainedas a condensate. The contents of the flask were cooled, the solidsseparated by filtration, washed with isooctane, then with water, anddried. The product weighed 238 g. and showed a NE. of 116.3.

Example 8 10 g. of Se in 2400 cc. of trichlorobenzene were oxidized asdescribed in Example 6 in the flask described in Example 5. 100 g. of2,6-dimethyl naphthalene were introduced into the flask causing the SeOto be immedately reduced to Se, as shown by the disappearance of thesolid SeO slurry and the formation of a clear red solution.

N0 gas was introduced at the rate of 1.5 to 1.6 g. per minute until 24g. of condensed water was collected from the exit gases. The temperaturewas permitted to rise from 185 C. to 200 C. at the end of the reaction.The exit gases were practically colorless showing practically completereduction of N0 to NO. The latter was dried by contact with silica gel,then mixed with air and passed through silica gel to reoxidize it to N0for reuse in the cycle.

The contents of the flask were cooled, the solids separated byfiltration, washed with isooctane and then with water, and dried. Theproduct weighed 111 g. and showed a NE. of 154; it was a very lightcream color.

2 It was treated with NaOCl as described in Example 6, the pure productshowing a NE. of 109.

Example 9 A solution of 200 g. of 2,3-dimethyl naphthalene and 4.5 g. ofSe in 2030 g. of trichlorobenzene was heated to 185 C. in the flaskdescribed in Example 5. N9 gas was introduced at the rate of 2 g. perminute while maintaining the temperature between 185 and 192 C. until 58 g. of water was recovered in the condenser from the exit gases, whichremained practically colorless until substantially the end of thereaction, showing that the reduction of the N0 was virtually complete.The contents of the flask were cooled, the solids recovered therefrom byfiltration, washed with isooctane and then with water, and dried. Theproduct weighed 184 g. and showed a NE. of 98, which is theoretical for2,6-dicarboxy-naphthalene anhydride.

Example 10 2-rnethyl-6-acetonaphthalene was boiled with an excess of 2%NaOCl in NaOH aqueous solution to form 2- methyl-6-carboxy naphthalene.20 g. of the product was dissolved in 590 cc. of trichlorobenzene withthe addition of l g. of Se and maintained at 190 C. Oxidation wasconducted by passing in N0 gas as described above. There was obtained g.of product having a NE. of 112 and being largely 2,6-dicarboxynaphthalene.

From the above examples it is clear that the alkyl naphthalenes may beoxidized wtih N0 gas at elevated temperatures to produce carboxylicacids with good yields. In the case of the dimethyl naphthalenes theproducts may show some coloration, indicating the presence of impuritieswhich for some purposes would be undesirable. However, when Se ispresent during the oxidation reaction, the crude products have a verylight color which is readily eliminated by the treatment with NaOCl inalkaline solution.

The concentration of the allryl naphthalene may vary considerably and ithas been found that up to about 25% is practical. Substituted alkylnaphthalenes are suitable provided that they do not interfere with thedesired reaction; for instance, one or more halogens, such as chlorine,or nitro groups may be present. The starting material may have otheralkyl groups than methyl as they are also amenable to the presentprocess. The process is susceptible of continuous operation withrecycling of solvent, Se, and N0 In another embodiment, the invention isdirected to the production of oxidation products of alkyl benzenes andderivatives thereof, and more particularly to the formation of monoandpoly-carboxylic acids therefrom.

Previously such alkyl benzenes as p-xylene were oxidized with dilutenitric acid at elevated temperatures and high pressures, up to about 580pounds per square inch to form terephthalic acid. In some cases thenitric acid was added in small increments and even dropwise in order toavoid the formation of undesirable byproducts but the process was longand tedious. Often air and oxygen was introduced into the reaction tofacilitate the oxidation. While some of these processes have been usedin commercial production, they suffer from disadvantages. For instance,the use of high pressures makes it necessary to provide specialequipment and the operation thereof becomes more expensive. The nitricacid is consumed in the process and there is generally little or norecovery possible of the nitrogen gases formed in the reaction, whichincreases the cost of production. Where the process requires a numberor" hours, the capacity of the apparatus is relatively small so that agreater investment is required for a given output. Acid-proof equipmentis necessary to avoid corrosion.

The present invention is intended and adapted to overcome thedisadvantages inherent in prior processes for the production of aromaticcarboxylic acids from alkyl benzenes, it being among the objects thereofto provide a process which is rapid and economical, wherein the yieldsof products are high, and the recovery thereof is simple.

It is also among the obiects of the invention to devise a process forthe stated purpose which is easily controlled, not requiring highlyskilled opera-tors nor special equipment.

it is further among the objects of the invention to provide an operationin which there is practically complete recovery of the oxidizing agentand in which the products of the reaction are in substantially purestate.

In practicing the present invention the starting material is an alirylbenzene, usually having one or more methyl groups attached to thebenzene ring although other alkyl groups having up to 6 carbon atoms ormore, are equally suitable in the process. The starting material isdissolved in a solvent which is inert to N0 at the elevated temperaturesutilized in the reaction. It has been found that various chlorinatedbenzencs are eminently suitable but preferably those having 2. to 4chlorine atoms are used, although higher or lower chlorinated benzenesmay be used. Also usable under certain conditions are such solvents asnitrobenzenes, diphenyl ether, chlorinated diphenyls, and others whichare sufiiciently inert and have boiling points above the temperaturesinvolved in the reaction.

A considerable range of temperatures may be used in the reaction. Theminimum temperature for commercial operation is about C. and preferablyit ranges from about to 200 (1., although higher temperatures up to thedecomposition points of reactants and products of reaction may be used.The temperatures are above those at which N 0 is largely dissociatedinto N0 as the latter is the effective oxidizer. In the reaction the N0is substantially completely reduced to NO making the recovery thereofand reconversion to N0 a simple and complete operation, so that it isrecycled with practically no loss. This is an important aspect of theinvention from the commercial and economic aspect.

Generally, the operation of the present process may be exemplified bythe oxidation of p-xylene to produce terephthalic acid or itsintermediate p-toluic acid. The pxylene may be dissolved intrichlor-obenzene to provide a 5% to 20% solution. l60-170 C. atatmospheric pressure and N0 gas bubbled through the solution. As theoxidation proceeds, the exothermic heat developed is removed by coolingso as to maintain the desired temperature. The N0 is reduced practicallycompletely and the exiting N0 gas is substantially colorless. P-toluicacid is formed. The temperature is then raised to about 19()200 C. andthe operation is continued until the conversion to terephthalic acid iscomplete. Since it is insoluble in the medium it may be removed from theslurry, washed and dried. The initial heating may be to about 19G200 C.whereby the terephthalic acid is formed directly.

Because of the physical condition of the slurry, the separation ofterephthalic acid may be conducted hot in a centrifuge equipped with astainless steel screen. The filtrate contains both p-xylene and p-toluicacid and is returned to the reaction. Usually the N0 is introduced in asteady stream, and at the temperature of the reaction vaporization ofp-xylene and trichlorobenzene takes place, and water vapor is formed;they are condensed and the p-xylene and tricluorobenzene are returned tothe reaction, usually continuously. Fresh p-xylene may be introduced atthe rate at which it is oxidized and the NC; at the rate at which it isreduced. This makes possible a continuous operation.

The efiiciency of consumption of N0 sets the production rate of theterephthalic acid and the rate of removal of the slurry determines theslurry density in the reactor. It is desirable to keep this density lowand the rate of The solution is heated to about removal of the slurry isgaged by this factor, whereby the operation can become practicallyautomatic.

As indicated above, the reaction may be in two stages, which areconducted in separate reactors. In such case the p-xylene escaping fromthe first reactor is condensed and refluxed into it. The reactionmixture in this reactor containing p-toluic acid, p-xylene andtrichlorobenzene may be continuously withdrawn and introduced into thesecond reactor. Any p-xylene stripped from the second reactor iscondensed and returned to it. The slurry is removed from this reactor,filtered, and the filtrate returned to the first reactor. Theconcentration of p-toluic acid remains below its saturation pointthroughout the process. The substantially pure terephthalic acid iswashed with the inert solvent to remove p-toluic acid and then withnaphtha or benzene to remove the solvent.

Because of the high efliciency of the reduction of the N it becomesfeasible to reconvert it by a simple and economical step. The effluentgases which are free from oxygen are cooled to condense the water vaportherefrom; then the gases may be further dried by passing through silicagel. Since NO which is free from N0 is insoluble in water and there isno oxygen present, there is no loss of NO in this step. The stream ofwater-free gas containing NO is mixed with dry air or oxygen and passedthrough a bed of silica gel which is cooled to absorb the heat ofoxidation generated, and the N0 is absorbed onto the silica gel.Thereafter the silica gel is heated to drive off the N0 which isreturned to the cycle. The N0 may be condensed for storage or it may befed directly into the reactor, thus completing the cycle.

The apparatus used in the following examples is essentially a 3-literresin flask equipped with a stirrer and having baflies along the sidesthereof. An inlet for N0 reaches near the bottom of the flask andanother similar inlet is provided for a solution of the alkylbenzene. Anoverflow near the top provides for the removal of the oxidized mixture.A- reflux condenser at the top of the flask returns condensed vapors(other than water) to the flask for reuse in the reaction.

The following are specific examples of the operation of the inventionwithout limiting the scope thereof.

Example 11 200 g. of p-xylene were dissolved in 2000 cc. oftrichlorobenzene heated to 152 C. in the flask. Gaseous N0 was admittedat a rate of about 1.8 g./min. for a period of about 2 hours duringwhich time the temperature rose from 152 C. to 167 C. without externalheating. During this period some p-xylene distilled over and wasreturned to the reactor at the end of the period. Also during this eriodpractically all of the N0 introduced into the reactor was used upleaving very little to appear in the exit gases. 28 g. of watercondensate were removed from the exit gases. The rate of N0 introductionwas then increased to about 2 g./min. and the temperature raised slowlyto 185 C. over a period of 50 minutes during which time the smallamounts of p-xylene that distilled over were returned to the reactor andat the end of which time solids formation appeared in the reactor. Therate of N0 admission was maintained for an additional 2 hours over whichtime the temperature was slowly raised to 197 C., and the color of theexit gases showed increasing proportions of N0 and considerable solidsaccumulated. The contents of the reactor were then removed, cooled, andthe solids separated by filtration, washed with isooctane and dried. Theweight of the dry solids was 243 g. and the NE. was 87.8 The solidscontained 86% terephthalic acid and 14% p-toluic acid.

Example 12 200 g. of p-toluic acid were dissolved in 2000 cc. oftrichlorobenzene heated to 190 C. in the flask. Gaseous N0 wasintroduced at a rate of 1.8 g./min. for a period of 30 minutes at whichtime solids appeared in the reactor 10 and the temperature had risen to197 C. The reaction was carried on for a further 2 hours during whichtime the color of the exit gases showed a rise from practically no N0content to a dark brown and the temperature rose to 200 C. The reactorwas then emptied, the contents cooled, the solids separated byfiltration, washed with isooctane and dried. The weight of the drysolids was 146 g. and showed an N.E. of 34.4.

Two things become apparent from the above examples. The first is thatterephthalic acid is quite insoluble in the hot inert solvent whereasthe p-toluic acid is very soluble in it. This permits the continualremoval of substantially pure terephthalic acid on one hand and therecycle of the solution of p-toluic acid on the other, giving very highyields of terephthalic acid from p-xylene or p-cymene. The second thingis that in the early stages of the batch runs, the exit gases from thereactor are comparatively free of N0 This completeness of reaction ofthe N0 is a function of the concentration of the organic reactants insolution. By proper recycle of p-toluic acid and addition of p-xylene itis possible to maintain the reactant concentration to the point wheresubstantially all of the N0 is reacted. The efliciency of utilization ofN0 is also a function of the design of the equipment used for contactingthe N0 gas with the reactant solution.

Example 13 200 g. of p-cymene were dissolved in 2000 cc. oftrichlorobenzene and heated to 170 C. in the flask. Gaseous N0 wasintroduced at a rate of 2 g./min. for 2% hours durirrg which time thetemperatures rose to 180 C. and 53 g. of water condensate wereaccumulated. The exit gases were practically colorless. The amount ofwater trapped points to the probability that the isopropyl group is thefirst to be oxidized. The N0 introduction was continued at a rate of 2g./ min. at temperatures between 182 and 186 C. for another 2% hoursduring which time the effluent igases became colored showing the escapeof some N0 The contents of the flask were cooled, filtered, the solids,largely terephthalic acid, washed with isooctane and dried. The weightof the dry solids was g. and their NE. was 84.8.

Example 14 200 g. of pseudocumene were dissolved in 2000 cc. oftrich'lorobenzene and heated to C. in the flask. Gaseous N0 wasintroduced at a rate of 2 g./min. for 2 hours during which time thetemperature rose to 181 C., 30 g. of Water condensate were accumulatedand the effluent gases were colorless. The N0 introduction was continuedat the same rate and the temperature slowly raised to 192 C. over anadditional 5 hours, during which time the efliuent lgases became coloredshowing the escape of someNO The contents of the flask were cooled,filtered, the solids washed with isooctane and dried. The dry solidsweighed 211 g. and had a NE. of 74.2, showing this to be largelytrimellitic acid.

Other alkyl benzenes may be treated in a manner similar to the aboveexamples. For instance, orthoand meta-xylenes have been oxidized by theprocedure of Example 11 to form phthalic and isophthalic acids,respectively; similarly in accordance with Example 12, orthoandmeta-toluic acid have been oxidized to phthalic and isophthalic acids,respectively. By following Example 12, p-nitro toluene has been oxidizedto p-nitro benzoic acid; and in accordance with Example 13, p-chlorotoluene has been oxidized to p-chloro benzoic acid. Similarly, thereactions may be conducted for partial oxidation so that not all of thealkyl groups are converted to carboxylic groups. Isomers of thepolyalkyl benzenes of the specific examples may be similarly treated,such as oxidation of durene to pyromel'litic acid. Such alkylatedbenezenes which have substituent groups, other than alkyl and which donot interfere with the oxidation, are usable in the process. In somecases, the anhydrides of the acids may be produced.

In another embodiment, "the invention is directed to the production ofanthraquinone from anthracene, and more particularly by a processwherein N is used as the oxidizing agent.

In the past it has been proposed to oxidize anthracene to the quinone bydissolving it in acetic acid and contacting the solution with nitrousgases. This process was not sufiiciently effective to warrant actualproduction, various oily by-products were formed, nitration took place,and recovery of anthraquinone from the solvent and by products was quitediiiicult and expensive.

Another prior process provided a mixture of anthracene with nitrousoxide, and basic zinc or copper oxides, the mixture being heated to250350 C. in a stream of air or oxygen, which was the oxidizing medium.The anthraquinone formed was s-ublirned and recovered. This process wasnot practical, the reaction was slow, and it resulted in a low yield.The long time at the high temperature caused decomposition. Theconstituents, such as anthracene and the metal oxides, being solidsthere was insuficient contact between them, the nitrous oxide and theoxygen, as a result of which conversion to anthraquinone was incomplete,so that it could not be used for commercial production. To prevent theformation of by-products, it was attempted to dilute the mixture withpowdered pumice, asbestos or glass wool and to introduce zinc dust orlead oxide to take care of nitric acid formed in the operation, but thisdid not lead to a successful process as the yields were poor.

The present invention is intended and adapted to overcome thedisadvantages inherent in prior processes for the oxidation ofanthracen-e to anthraquinone, it being among the objects thereof todevise a process for the stated purpose which is simple and highlyefiective.

It is also among the objects of the invention to provide a process whichis rapid, gives a high yield of product of high purity, which is adaptedfor continuous operation, and which does not require expensive orcomplicated equipment nor highly skilled operators.

It is :further among the objects of the invention to so conduct thereaction using N0 as the oxidizing medium that the resulting nitrogenousgases may be practically all recovered in a state for reconversion to N0for recycling inthe system without loss in side reactions.

In practicing the invention, a solution is made of anthracene in asolvent which is inert to N0 at the elevated temperatures utilized inthe reaction. Most organic solvents react with N0 under the relativelyhigh temperature conditions, but it was found that chlorinated benzenesare excellent for the purpose. Among these compounds the monototetra-chlorobenzenes are quite suitable, including the mixedtetrachlorobenzenes. The solvent should remain liquid at the reactiontemperatures which are those at which N 0! is largely dissociated. Othersolvents which have been found usable although they may show somereaction at the higher temperatures used include nitrobenzene, diphenylether and chlorinated diphenyls. Still other solvents may be used in theoperations More specifically, the temperature should be above about 125140 C. and preferably above about 180 C., and up to the point wheredecomposition may become a factor. The oxidizing medium is N0 in thegaseous state. A solution containing, say 5% to of anthracene is held atthe desired temperature and an amount of N0 sutlicient for the formationof anthraquinone is introduced in finely divided form into a reactorprovided with adequate agitation at such a rate that substantially allthe N0 is reduced to NO and the exit gases are practically free lfI'OITlthe red-brown color of N0 The reaction is rapid and goes to practicalcompletion in a matter of minutes. -It lends itself readily to acontinuous operation. The anthraquinone produced is of high quality andalmost quantitative yields are obtained.

After the completion of the reaction, the solvent containing thesuspended anthraquinone is cooled and filtered. The product is washedwith a volatile organic liquid which does not react in order to removethe inert solvent, and the product is dried.

With the present process, the recovery of the N0 used in the operationmay be recovered for re-use. Because the residual gases from thereaction are practically free from N0 the NO can be treated forsubstantially complete regeneration of N0 The eflluent gases are cooledand freed from water produced in the reaction as well as vapors ofsolvent, by passing through a condenser at the proper temperature. Thegases leaving the condenser are further dried by being passed throughsilica gel, thus avoiding any loss of nitrogen gases since NO isinsoluble in water and practically no oxygen or'NO is present. Then theN0 gas is mixed with oxygen or air in suflicient amount to oxidize NO toN0 and passed into a cooled bed of silica gel which catalyzes theoxidation and adsorbs the N0 formed. To release the N0 for re-use thesilica gel is heated.

Example 15 A solution of 178 parts by weight of anthracene in about 2000parts by weight of trichlorobenzene are placed in a 3-liter resin flaskfitted with agitator, battles, gas inlet and outlet tubes. Gaseous N0 atabout C. is introduced at the bottom of the flask and is finelydispersed by the action of the agitator and battles. The rate ofintroduction is about 2 parts by weight per minute, which requires about90 minutes. After the accumulation of about 16 parts by weight of water,the exit gases begin to show a brownish tinge showing the reaction to becomplete. The reaction mixture is then cooled to room temperature, theanthraquin-one filtered oil and washed with iso-octane to remove theresidual solvent, and then dried. The product is 188 parts by weighthaving a melting point of 28028l C.

The preceding example does not limit the invention as various changesmay be made in the details of the operation. For instance, the rate ofaddition of N0 may be greatly increased in properly designed apparatusand the reaction completed in a few minutes. Higher temperatures alsoreduce the time of reaction. in the start of the process the solution ofthe anthracene may be heated to the desired temperature to initiate thereaction and then, as the exothermic heat develops, cooling of thereaction vessel may be resorted to. Other inert solvents, such as themixed tetrachlorobenzenes may be used. The concentration of theanthracene may be varied. Higher temperatures up to about 250 C. arefeasible if precautions are taken to avoid loss of solvent anddecomposition. The operation may be conducted under super-atmosphericpressures. Various other details may be altered without departing fromthe principles involved herein.

Prior processes using nitrogen compounds were unable to produceanthraquinone free from nitrogen-containing products, whereas a verypure anthraquinone results from this invention. In the present processthe residual NO gases being substantially free from N0 are readilyreoxidized for return to the cycle, thus eifecting economy in operation.The yield of anthraquinone is quite satisfactory.

In another embodiment, the invention is directed to the production ofselenium dioxide (SeO from selenium, and more particularly to a processwhich is highly ellicient and economical.

Selenium dioxide, as such or in the form of selenious acid is a knownoxidizing agent for organic and inorganic substances. For instance, theacid will oxidize sulfurous acid to sulfuric acid. In producing SeO inthe prior art, elemental Se was heated in concentrated nitric acid tocause the Sc to become oxidized. Too much nitric acid was consumed inthe operation and was wasteful of nitric acid. Also, great care had tobe exercised to prevent explosions.

In another prior art process, the Se was treated with air or oxygen athigh temperatures in the presence of traces of nitrogen oxides ascatalyst, the resulting combustion producing SeO The oxidation tookplace at temperatures well above 217 C., the melting point of Se, andthe SeO was recovered by sublimation which introduced difliculties. As aresult, the production of large quantities of SeO by either of the abovemethods left much to be desired.

The present invention is intended and adapted to overcome thedifficulties inherent in the prior method for producing SeO from Se, itbeing among the objects thereof to provide a process which is simple,rapid, does not require complicated or expensive equipment, and gives ahigh yield of SeO in pure form.

It is also among the objects of the invention to provide a process forthe stated purpose in which the reaction to produce SeO may proceedsimultaneously with the oxidation of an organic substance by the SeO soformed.

It is further among the objects of the invention to devise a process inwhich gaseous N is used to oxidize the Se and the NO formed thereby maybe recovered as N0 for reuse.

The invention is based upon the fact that Se is soluble to some extentin certain organic solvents, more particularly in halogenatedhydrocarbons, specifically chlorinated aromatic compounds such aschlorobenzenes carrying from 1 to 6 chlorine atoms. While the solubilityof Se is very small at room temperatures, it increases to a substantialdegree at temperatures at from about 130 C. to 230 C. Other chlorinatedsolvents are suitable including as the homologues of the chlorobenzenes,the chlorinated diphenyls, particularly those containing about 20%chlorine, and nitrobenzene. Still other solvents which are inert to N0and SeO at the temperatures of the reactions may be used in the process.

It has now been found that the solution of Se in the above identifiedsolvents at elevated temperatures is readily reacted with N0 whereby Se0is rapidly formed and is precipitated as it is insoluble in thesesolvents and a slurry thereof is formed which is easily filterable, theeflluent NO formed in the reaction being practically colorless untilsubstantially all of the Se has been consumed when it begins to take onthe reddish color of N0 The NO may be dried to remove traces of waterand then mixed with oxygen -or air and passed through silica gel toregenerate N0 which may be recycled.

Since the solubility of the Se is limited, additional Se may beintroduced simultaneously with the N0 in proper proportions, orpreferably a considerable excess of Se may be introduced into thesolution and the reaction proceeds with the continuous dissolving of Seand precipitation of SeO until the Se has become exhausted. Whenundissolved Se no longer remains, the slurry is filtered while hot sothat there is no contamination of the SeO The filtrate is reused for thefurther production of SeO Thus the reaction lends itself to continuousoperation and close control.

The SeO being in a readily filterable state, is usually separated in acentrifuge to form a cake which has a low solvent content. The residualsolvent is washed out with Sefree hot solvent, and the latter is washedout with a low boiling solvent such as benzene or naphthas.

In dissolving the Se in the solvent as the temperature is increased fromroom temperature to say about 190 to 200 C., the colorless solutionpasses through yellow to reddish and at the saturation point it is deepred. High er temperatures than 200 C. may be used but such temperaturesshould be below those at which the solvent vaporized under theconditions of the operation. It is preferred to conduct the oxidationbetween 160 and 200 C.

The following are specific examples of the operation of the invention:

Example 16 A 3-liter resin flask is fitted with a downdraft propelleragitator near the bottom thereof. An inlet is provided for introductionof solvent and Se. A gas inlet tube extends down to a point just abovethe propeller. An exit tube is connected to a reflux condenser adjustedfor the condensing and return of vaporized solvent.

2000 cc. of trichlorobenzene are placed in the flask and 50 grams of Seintroduced therein. Heat is applied to raise the temperature to C. N0 isintroduced at the rate of 1.5 grams per minute, the eflluent gasremaining practically colorless showing practically complete reductionof the N0 to NO until the Se in solution was fully oxidized. The end ofthe reaction is indicated by the reddening of the exit gases. The slurryof SeO is removed, and filtered in a dry atmosphere, washed withtrichlorobenzene, then with benzene, and finally dried, yieldingsubstantially pure SeO Example 17 In the apparatus of Example 16, 2000cc. of trichlorobenzene are introduced containing 14 grams of SeO It isheated to -200 C. and ethylene gas is passed into the slurry. Thereaction is very rapid as shown by the fact that the solution begins toredden immediately upon the introduction of the ethylene, showing thatSe is formed and is in solution. In about 10 to 15 minutes all of thesolid SeO in suspension disappears and a clear red solution is obtained.The water formed in the reaction is condensed and it contains glyoxalwhich is the product of the reaction.

Example 18 In the apparatus of Example 16, there is introduced into theflask a mixture of 2000 cc. of trichlorobenzene and 10 grams of Se. Onlya part of the Se goes into solution as the temperature is raised to 195C. forming a deep red solution. Ethylene and N0 are flowed in throughthe gas inlet, the N0 at such a rate that it attacks the Se inpreference to attacking the ethylene, so that oxidation thereof takesplace by the SeO and not by the N0 Therefore, the formation ofby-products by direct oxidation of ethylene by N0 is largely avoided.The water formed in the reaction contains glyoxal.

Various types of organic compounds may be oxidized as set forth above,such as acetaldehyde to glyoxal, benzyl alcohol to benzaldehyde, camphorto camphorquinone, diphenylmethane to benzophenone, and fiuorene tofluorenone. When the products of the oxidation are volatile at thetemperature of the operation, they may be removed from the solution byevaporation. In other cases they may be recovered by other means, suchas cooling and filtration. In certain instances the organic reactant maybe added to the inert solvent bearing the Se during the introduction ofthe N0 where N0 does not form undesirable reaction products with theorganic reactant or a product of reaction in the presence of Se or SeOThere are several advantages flowing from the invention. The reaction ofN0 with dissolved Se is very rapid and complete. SeO formed in thereaction is readily separable from the mixture. The yield of SeO is highand it is in substantially pure state. No complicated equipment isinvolved and the control of the conditions is simple, so that relativelyunskilled chemical workers may operate the process with goodefiflciency. Cycling the NO to re-form N0 is quite valuable incommercial operations in reducing cost of processing.

The invention claimed is:

1. A method of producing SeO Which comprises: dissolving elemental Se inan organic solvent inert to N0 and SeO at elevated temperature, saidsolvent being selected from the group consisting of halogenatedaromatics and nitrobenzene maintaining a temperature above about 130 C.and below the vaporization temperature of the solvent, and introducingN0 into said solvent containing dissolved Se, whereby SeO is formed byoxidation of Se with N0 and is precipitated as a slurry.

2. Method according to claim 1 wherein said temperature is between about130 C. and 230 C.

3. A method according to claim 1 characterized in that the Se0 formed isseparated from said solvent and Washed.

4. A method according to claim 1 characterized in that an excess of Sein undissolved form is present during at least the initial stages ofsaid oxidation.

5. A method according to claim 1 characterized in that an organiccompound oxidizable by SeO is present during said oxidation said organiccompound being selected from the group consisting of ethylene,acetaldehyde, benzyl alcohol, camphor, diphenylmethane and tfiuorene.

6. A method according to claim 5 characterized in that the rate ofadditon of N0 is such that it attacks the Se but is insufiicient inamount to react with said organic compound.

References Cited by the Examiner UNITED STATES PATENTS 2,616,791 11/1952Rosernan et al. 23-439 OTHER REFERENCES Barnes, Journal of the IndianChemical Society, volume 9, 1932. pages 329, 333. V

Booth et al., Editors, Inorganic Synthesis, volume 1, McGraW-Hill BookCo. Inc., New York, 1939, pages 117-118.

OSCAR R. VERTIZ, Primary Examiner.

BENJAMIN HENKIN, Examiner.

H. T. CARTER, Assistant Examiner.

1. A METHOD OF PRODUCNG SEO2 WHICH COMPRISES: DISSOLVING ELEMENTAL SE INAN ORGANIC SOLVENT INERT TO NO2 AND SEO2 AT ELEVATED TEMPERATURE, SAIDSOLVENT BEING SELECTED FROM THE GROUP CONSISTING OF HALOGENATEDAROMATICS AND NITROBENZENE MAINTAINING A TEMPERATURE ABOVE ABOUT 130*C.AND BELOW THE VAPORIZATION TEMPERATURE OF THE SOLVENT, AND INTRODUCINGNO2 INTO SAID SOLVENT CONTAINING DISSOLED SE, WHEREBY SEO2 IS FORMED BYOXIDATION OF
 5. A METHOD ACCORDING TO CLAIM 1 CHARACTERIZED IN THAT ANORGANIC COMPOUND OXIDIZABLE BY SEO2 IS PRESENT DURING SAID OXIDATIONSAID ORGANIC COMPOUND BEING SELECTED FROM THE GROUP CONSISTING OFETHYLENE, ACETALDEHYDE, BENZYL ALCOHOL, CAMPHOR, DIPHENYLMETHANE ANDFLUORENE.